Hi Julian, I've been using Canon SLR's (film from 1980 to 2004/5) and then the now venerable Canon D60 DSLR from then onwards.
I boarded the M 4/3's bus in 2009, when I bought a Panasonic GF1 with it's 14/45 kit lens and then later on got myself the 20mm f1.7
Recently I've been using a brand new (borrowed) Panasonic 45-175 HD, X series zoom f4/5.6 zoom lens.
This translates into a 35mm equivalent of 90 - 350mm.
My GF1 has a menu setting called Expert optical zoom,which when switched on and used with the Medium (M)Fine picture size extends the Optical zoom reach by almost 1.5 times to 130 - 500mm.
Now imagine this range in a small, internally focussing lens and a small M 4/3's body and you will bless the small form factor and lightness.
Nikons are great cameras, especially in their Low ISO performance. Having said that, if you read the DP Review conclusion, they opine that the OMD EM5 with it's new 16 Mp sensor is so good that the difference in quality between a M4/3 and an APS sensor is almost non-existant and that to get any appreciable jump in quality, one would have to consider a Full Frame camera.
I have used the GF1 with the kit and 20mm lens combo extensively on my travels for 2 years and raelly dont miss a DSLR, even though its low light performance and resolution isnt as good as the OMD.
I carry only those 2 lenses and dont have to lug an SLR around from 9am to 11 midnight on my travels.
Although my GF1 is not an ideal tool for " birding" as it lacks an eyelevel viewfinder and has only 3 or 4 fps to offer, I've been using the 45-175 zoom to shoot some bird pics.
Please keep in mind that the GF1 has "only" 12 Mp res and no IBIS, where as the OMD has 16 Mp and 5 axis IBIS !
If you are considering a DSLR, please also look at the new Canon 650 D with a Hybrid 18 Mp PDAF/CDAF sensor and a swivel LCD rear screen and other goodies, such as the new Canon 40mm pancake lens !
You may wish to see some pics from my trip to Greece, posted earlier in May and June this year to get an idea of the output of the GF1 and the 2 lenses.